Thus far, erasable optical discs such as magneto-optical discs, read after write optical discs, non-erasable ROM type optical discs, and so forth have been known. When such optical discs are used for storing digital data, it is necessary to place an optical pickup to a designated track position, namely target track as quick as possible so as to shorten the access time.
To move the optical pickup to the radial direction of the optical disc, for example, a linear motor is used. Conventionally, to shorten the seek time of the target track, the optical pickup is controlled so that it is moved at a high speed and the moving speed nearly becomes 0 at the target track, thereby enabling the tracking control to be performed immediately at the target track.
FIG. 1 shows a seek circuit proposed by the applicant of the present application in U.S. patent application Ser. No. 234,197/1988. In FIG. 1, reference numeral 2 denotes a counter. The counter 2 is cleared with a start signal SC1 sent from a control circuit 1. The counter 2 incrementally or decrementally counts (up or down) in accordance with an up-down control signal SC2. The counter 2 counts a track traverse signal Stv which is generated whenever a track is traversed by the optical pickup during a seek operation.
A count value CT of the counter 2 is sent to a D/A converter 3 and then converted into an analog signal E. The analog signal E is sent to an input terminal of the switch circuit 5. In addition, the analog signal E is sent to the other input terminal of the switch circuit 5 through an inversion circuit 4. The switch circuit 5 is switched with an output signal of the control circuit 1 in accordance with the seek direction. An output signal of the switch circuit 5 is sent to a subtraction circuit 6. An output signal of the subtraction circuit 6 is sent to an actuator 8 through a drive circuit 7. The actuator 8 is a linear motor or the like.
An optical pickup (not shown in the figure) is moved in the radial direction of an optical disc along with the actuator 8. A speed sensor 9 for detecting the moving speed of the optical pickup is provided. An output signal Sv of the speed sensor 9 is fed back to the subtraction circuit 6. In the case of the linear motor, the speed sensor is magnetically composed. In other words, the moving speed of the optical pickup is detected with a voltage induced by a change of magnetic flux of a coil moved along with the optical pickup.
In the conventional structure shown in FIG. 1, the control circuit 1 generates the up-down control signal SC2 (see FIG. 2A) for controlling the counter 2 so that the counter 2 counts up while the optical pickup is moved from the current track to TR1 which is half the value of TR2 which is the difference between the target track and the current track; and the counter 2 counts down while the optical pickup is moved from TR1 to TR2. Thus, the D/A converter 3 generates the analog signal E which goes high after the seek operation is started and the counter 2 is cleared with the start signal SC1 until TR1 and which goes low from TR1 to TR2 as shown in FIG. 2B.
In addition, the speed sensor 9 generates the output signal Sv as shown in FIG. 2C. While the actuator 8 is being moved with the output signal Sv of the speed sensor 9 from the current track to TR1, which is half the value of the target track TR2, the optical pickup is moved at a high acceleration. When the optical pickup is placed at the target track TR2, the moving speed thereof becomes 0.
There has been proposed an alternative method in which the analog output signal E from the D/A converter 3 is converted into its square root value rather than directly using the analog output signal E as the drive signal so as to smoothly decrease the speed of the actuator 8 at the target track TR2.
In the conventional seek circuit, it is necessary to provide the speed sensor 9 for obtaining the speed information of the actuator 8. However, when the linear motor is used, the speed sensor 9 is not necessary to generate the drive force. If the speed sensor can be omitted, the drive force can be further increased. In addition, besides the linear motor, when the speed sensor is provided along with the actuator, the apparatus will disadvantageously become large and require more parts.